1. Field of the Invention
The present invention relates to a microorganism having an enhanced L-valine productivity and a method for producing L-valine using the same.
2. Description of the Related Art
L-amino acids are used in human medicine, in particular, in the pharmaceutical industry, food industry and animal nutrition or the like. Particularly, branched-chain amino acids refer to three amino acids among nine essential amino acids: L-valine, L-leucine, and L-isoleucine. Unlike other amino acids, which are mostly metabolized in the liver, branched-chain amino acids are mainly metabolized in the muscle tissue and serve as an energy source during exercise. As branched-chain amino acids are known to play an important role in muscle maintenance and growth during exercise, their use is growing. Specifically, L-valine has been used as a feed component, because it was reported that L-valine has high reducing power and serves to improve lactational performance of sows. L-valine has also been used in infusion solutions and amino acid complexes for medical purposes, and in health supplements and beverage additives.
A microorganism used for the production of L-amino acids is represented by coryneform bacteria, particularly Corynebacterium glutamicum. Due to the high importance of coryneform bacteria in industrial production, methods for producing L-amino acids using the microorganisms are continuously undergoing improvement. For example, improvements are being made to improve the method relating to stirring and introducing oxygen, or compositions of the culture media such as sugar concentration during fermentation. To improve the L-amino acid productivity of these microorganisms, selection and mutant selection methods are widely employed. For example, there is a method of selecting and using microorganisms that are resistant to antimetabolites such as an isoleucine derivative, isoleucine hydroxamate (Kisumi M et al., (1972) Journal of Bacteriology 110: 761-763), an L-valine derivative, 2-thiazole alanine (Tsuchida T et al., (1975) Agricultural and Biological Chemistry, Japan 39: 1319-1322) or a leucine derivative, α-aminobutyrate (Ambe-Ono Y et al., (1996) Bioscience Biotechnology Biochemistry 60: 1386-1387), or auxotrophic for metabolites having regulatory relevance and produce L-amino acids (Eva Radmacher et al., (2002) Applied and Environmental Microbiology, Vol. 68 p. 2246-2250).
Meanwhile, one of the branched-chain amino acids, L-valine is biosynthesized in a microorganism, starting from pyruvic acid via acetolactic acid, dihydroxy isovaleric acid, and ketoisovaleric acid. These intermediate metabolites are produced by catalytic activities of acetohydroxy acid synthase, acetohydroxy acid isomeroreductase, dihydroxy acid dehydratase, and transaminase B. However, these enzymes are also involved in L-isoleucine biosynthesis starting from ketobutyric acid and pyruvic acid, and L-leucine is also biosynthesized from the intermediate metabolite, ketoisovaleric acid via 2-isopropylmalic acid, 3-isopropylmalic acid, and ketoisocaproic acid. Therefore, since the enzymes used in the biosynthetic pathways of the branched-chain amino acids, namely, L-valine, L-isoleucine, and L-leucine are identical, it is difficult to produce only one of the branched-chain amino acids by industrial fermentation. Additionally, feedback inhibition by the final product L-valine or derivatives thereof occurs, which makes it difficult for industrial mass production of L-valine.
To solve these problems, many studies have been made to develop L-valine-producing microorganisms having resistance to L-valine or derivatives thereof for the production of L-valine, and they are exemplified by a method of using a microorganism having resistance to D, L-amninobutyric acid (Japanese Patent Laid-Open No. 563-160592), a method of using a microorganism that is resistant to thiazole alanine and auxotrophic for leucine, isoleucine, or threonine (Japanese Patent Laid-Open No. 552-116), a method of using a microorganism having resistance to amnioethylcysteine (Japanese Patent Laid-Open No. S58-2678), a method of using a microorganism having resistance to L-valine in an acetic acid-supplemented medium and having sensitivity to pyruvic acid in a glucose-supplemented medium (U.S. Pat. No. 5,521,074, Korean Patent No. 1995-0005133), a method of using a microorganism having resistance to polyketide (Korean Patent No. 1996-0016871) or the like.
However, currently developed L-valine-producing microorganisms are resistant only to a single material or a limited material of L-valine or derivatives thereof, and thus there is still a need to develop L-valine-producing microorganisms having a resistance to various materials involved in the feedback control of L-valine biosynthesis.
For these reasons, the present inventors have made many efforts to develop microorganisms capable of producing L-valine in a higher yield than other conventional strains. As a result, they found that a mutant strain, obtained from a glutamic acid-producing microorganism, produces L-valine in a high yield, and has resistance to numerous L-isoleucine derivatives and L-valine derivatives, specifically, α-aminobutyric acid (ABA), α-hydroxyvaline (AHV), thiazole alanine (TA), and norvaline (NV), thereby completing the present invention.